The present invention relates to a frequency estimating system for estimating a center frequency of the carrier wave of a received signal.
As the frequency estimating system of this kind, one as shown in FIG. 6 is well known in the art. The illustrated frequency estimating system comprises a modulated component removing circuit 11, a Fourier transforming circuit 12, a power converting circuit 13 and a peak detecting circuit 14. A received signal having an unknown carrier frequency, for instance, is coupled via an antenna (not shown) to the modulated component removing circuit 11. The modulated component removing circuit 11 removes a modulated component from the received signal to obtain a non-modulated signal. The Fourier transform circuit 12 performs Fourier transform of the non-modulated signal and outputs the result as a frequency spectrum component. The power converting circuit 13 performs power conversion of the frequency spectrum component to feed a resultant power-frequency spectrum to the peak detecting circuit 14. The peak detecting circuit 14 detects the peak power level in the power-frequency spectrum and outputs a frequency corresponding to the peak power level as an estimated center frequency fp.
In the frequency estimating system shown in FIG. 6, it is possible to accurately estimate the center frequency when the received signal is not subjected to fading or the like. However, when the received signals exposed to rice fading or the like so that its carrier wave component has a spread frequency range, right center frequency may not be estimated.
Taking the rice fading as an example, when the received signal is subjected to the rice fading during transmission, a fading component subjected to external disturbance by irregular reflection and a straight component not subject to any external disturbance are combined, and the resultant signal is received. In a mobile communication system or the like, in which a terminal (i.e., a mobile terminal) is moved at a high speed and receives great influence of the Doppler effect, the fading component and the center frequency of the straight component may fail to be coincident with each other.
For example, in a GSM system the received signal has a carrier wave spectrum as shown in FIG. 7. As shown, the fading component and the center frequency of the straight component fail to be coincident. In the demodulation such that a received signal with a deviation between the fading component and the center frequency of the straight component, the best characteristic can be obtained when the received signal is inputted to a demodulator such that the center frequency of the fading component is zero. To make the fading component center frequency zero, estimation thereof is necessary. In the case of using the frequency estimating system as shown in FIG. 6, that is, in the case of detecting the peak of the carrier wave component, the frequency of the straight component is obtained as the estimated frequency (i.e., center frequency). In other words, when the received signal is subjected to fading during transmission, its carrier wave center frequency can not be accurately detected.